Frequency hopping is employed in some cellular, radio telecommunications systems, such as the Global System for Mobile Communication (GSM), to improve system performance. Frequency hopping improves system performance by introducing frequency diversity and interference diversity, and as a result, increases the capacity of a cellular network. In a radio telecommunications system that employs frequency hopping, typically a frequency hopping sequence is allocated to a mobile radio terminal at call setup. Frequency diversity is achieved by transmitting each radio telecommunications signal on the sequence of frequencies over time. Each radio signal is transmitted over a sequence of frequencies because radio signals are often subject to amplitude variations called Rayleigh fading. Generally, in any given instance Rayleigh fading negatively impacts radio signals carried on some frequencies more so than other frequencies. Thus, transmitting a radio telecommunications signal over the sequence of different frequencies can increase the likelihood that the signal will be received correctly, as it is unlikely that Rayleigh fading will significantly negatively impact each and every frequency over which the radio telecommunications signal is being transmitted. This benefit exists for signals containing redundancy that enables bit errors experienced during the Rayleigh fading dips to be corrected. Accordingly, signal quality is improved and overall system performance is enhanced.
In addition to fading, a radio signal is often subject to varying degrees of interference caused by traffic (e.g., from closely located mobile terminals or base stations) on the same frequency (i.e., co-channel interference) and traffic on an adjacent frequency (i.e., adjacent channel interference). If co-channel and/or adjacent channel interference is substantial, the signal quality associated with the radio signal may be severely impacted. In theory, frequency hopping, through the introduction of interference diversity, spreads the co-channel and adjacent channel interference among numerous mobile terminals so that the co-channel and adjacent channel interference experienced by a particular end-user is diversified. The overall effect is to raise the signal quality across the network, thereby improving overall system performance.
In order to avoid severe interference between closely-located mobile stations (e.g., mobile stations connected to the same base station) using the same set of frequencies, orthogonal frequency hopping sequences are allocated to these mobile stations. Two frequency hopping sequences S1 and S2 are orthogonal if S1(k)≠S2(k) for all time steps k. Orthogonality is depicted by the notation S1⊥S2. The two frequency hopping sequences S1 and S2 are partially orthogonal if the collision probability P(S1(k)=S2(k))=p for some 0<p<1, as depicted by the notation S1⊥p S2.
Frequency hopping sequences can be derived from a reference frequency hopping sequence that is established for the entire system or a part of the system, e.g. a cell. Typically (e.g., as in GSM), the reference frequency hopping sequence is a cyclic or pseudo-random sequence determined by cell specific parameters (e.g., Hopping Sequence Number “HSN”). That is, a mobile station, at handover or call set-up, is informed of the cell specific parameter determining the reference frequency hopping sequence and is in addition assigned mobile-specific parameters (e.g., an available frequency offset) associated with the cell in which the mobile station is operating. The mobile station hops through a sequence of frequencies that are, over time, offset from the reference frequency hopping sequence by a fixed amount that is equal to its assigned frequency offset. In accordance with the GSM standard, each frequency offset is referred to as a Mobile Allocation Index Offset (MAIO). Depending on the values of these two parameters (HSN and MAIO, for instance) and the parameter that clocks the frequency hopping sequences through time (the time division multiple access (TDMA) frame number, for example), frequency hopping sequences used by two mobiles may be either identical, orthogonal, or non-orthogonal but with random collisions, so that interference diversity is achieved. Allocating different constant frequency offsets to mobile stations at call setup or handover is an attempt to obtain orthogonality.
Commonly-assigned U.S. Pat. No. 6,233,270 discloses a mobile station hopping from one frequency to another as a function of the reference frequency hopping sequence plus a frequency offset hopping sequence which it has been assigned. The frequency offset hopping sequence is different in each of the synchronized cells, thereby creating interference diversity. U.S. Pat. No. 6,233,270 thus describes a method to obtain interference diversity between synchronized cells that have been allocated a same frequency hopping sequence.
Interference diversity within a cell can also be important for networks in which either intra-cell co-channel or intra-cell adjacent channel interference occurs. Assume, for example, a typical GSM frequency hopping method using two basic parameters: the hopping sequence number (HSN) and the frequency offset (MAIO) from the basic frequency hopping sequence. Frequency offset hopping sequences should be generated so that, depending on the input parameters, two frequency offset sequences are either orthogonal with variable frequency offset difference, or non-orthogonal with random collisions so that interference diversity can be provided by the frequency offset sequences alone (irrespective of the basic frequency hopping sequence). Commonly-assigned U.S. Pat. No. 7,421,005 describes a frequency hopping sequence generator system that generates variable frequency offsets to determine a frequency hopping sequence for use in communication between a mobile station and a network node in an effort to provide interference diversity within a cell based on frequency offset.
Recently, a technique called Multiple User Reusing One Slot (MUROS) is being discussed for use in TDMA type systems. MUROS allows two or more mobile terminals to share the same carrier frequency during the same time slot, both in the downlink and in the uplink. In a first MUROS approach, Quadrature Phase Shift Keying (QPSK) modulation is used in the DL (downlink), and two user signals are mapped to the real and imaginary parts of the baseband signal. The real part is call an I sub-channel, and the imaginary part is called the Q sub-channel. Under some conditions, the I and Q sub-channels are orthogonal, and therefore, named Orthogonal Sub-Channels (OSC). Another approach, called Adaptive Symbol Constellation, proposes using a hybrid quaternary modulation in the downlink (DL). The Adaptive Symbol Constellation concept is an extension of OSC. In the uplink (UL), Gaussian minimum shift keying (GMSK) modulation is used for both approaches. Two GMSK-modulated signals from two mobile stations are transmitted on the same timeslot and carrier frequency (or sequence of frequencies in case frequency hopping is deployed). On the receiver side, multi-user detection or interference cancellation techniques can be used to demodulate the two signals. Even though legacy mobile terminals are supported by the first approach, new mobile terminal types will still be required because a new training sequence set is introduced. Moreover, the frequency hopping standardized for GSM can be applied to the QPSK-modulated signal in the downlink and to each of the GMSK-modulated signals in the uplink. In that case, the two sub-channels will use the same frequency hopping sequence, and hence, the same frequency and timeslot at any given instant in time.
Thus, in MUROS, two (or more) mobile terminal communications share one radio resource. Even though OSC implies the existence of orthogonality, the two mobile terminal signals are not perfectly orthogonal because time dispersion on the channel (due to multipath propagation on the radio channel and filters in the transmitter and receiver) causes leakage between the I and Q sub-channels. For the downlink, this means the two mobile terminal signals interfere with one another. For the uplink, the phase difference between the two mobile terminal signals is random, and thus, orthogonality is not achieved even in the absence of time dispersion. The result of this lack of orthogonality is interference between the two sub-channels that degrades the performance (e.g., speech quality) for each user. Even if discontinuous transmission (DTX) is used, the inter-sub-channel interferer is sometimes present and sometimes not. Performance decreases in the time periods when the interferer is present.
Further, in a scenario in which OSC is used for only a subset of the channels in a cell (e.g., due to the current cell load, there is no need to multiplex two users on all channels), users on OSC channels will experience worse link quality (e.g., coverage) than users on non-shared channels.
Accordingly, there is a need for interference diversity in these MUROS type settings.